This invention relates to a process for preparing superficially porous macroparticles for use in chromatography and as catalysts or catalyst supports.
In chromatography it is customary to pass a mixture of the components to be resolved in a carrier fluid through a separative zone in a chromatographic apparatus. The separating or resolving zone generally consists of a material which is chromatographically sorptively active. Chromatographic apparatus generally employs packed columns of granular material. For analytical application the columns usually are of small internal diameter, while for preparative chromatography, larger diameter columns are employed. Support materials commonly employed for chromatography are granules having sorptively active surfaces or surfaces which have been coated with a substance which is sorptively active. Passing the mixture to be separated through the column results in repeated chemical interactions between the different components of the sample and the chromatographically active surfaces. Different compounds migrate at different speeds through the column because of these repeated, selective interactions. The separated components in the column effluent are generally passed through an analyzer or detector, for example a flame ionization detector in gas chromatography or an ultraviolet absorption detector in liquid chromatography, to determine when the resolved components emerge from the column and to permit the identification and quantitative measurement of each component.
Particles for use as catalysts or for catalyst supports should have overall size and porosity to permit ready access of reacting species to catalytically active sites within the particles.
It has long been recognized that superior chromatographic supports for liquid chromatography would consist of a plurality of discrete particles of regular shape, preferably spheres, having surfaces with a large population of superficial, shallow pores and no deep pores. The support granules should be regular and their surface characteristics readily controllable and reproducible. The same ready-access characteristics that make particles superior for chromatography also are desirable for catalysts and catalyst supports. Such particles have been very difficult to realize in practice with a result that the cost of superficially porous column chromatographic packings and particularly catalyst and catalyst supports has inhibited their use.
British Pat. No. 1,016,635 discloses a chromatographic support made by coating a particulate refractory solid on an impermeable core. The coating is accomplished by dispersing the coating material in a suitable liquid in a slurry. The cores are then coated with the slurry, withdrawn, and dried to remove the liquid. The result is a rather loosely held, mechanical coating of nonuniform, disoriented particles. These coated cores may be used as chromatographic supports.
Kirkland (Kirkland, J. J., "Gas Chromatography 1964," A. Goldup, editor, The Institute of Petroleum, London, W.1, pp. 285-300, 1965) has described the preparation of a chromatographic support by bonding successive layers of silica microparticles to glass beads by means of very thin fibrillar boehmite films. These coated cores may be employed as chromatographic or catalysts or catalyst supports.
Coated glass beads consisting of a single layer of finely divided diatomaceous earth particles bonded to the glass beads with fibrillar boehmite have also been described as a chromatographic support (Kirkland reference as above; Kirkland, J. J., Anal. Chem., 37, 1458-1461, 1965).
A method of preparing superficially porous particles by depositing colloidal inorganic particles of a given size and ionic charge from aqueous dispersion onto the surface of a solid, a single monolayer of microparticles at a time, and by repeating the process, to coat the surface with any desired number of monolayers, is described in Canadian Pat. No. 729,581.
U.S. Pat. No. 3,505,785 issued to Kirkland on Apr. 14, 1970, discloses a method of preparing superficially porous particles by first forming a coating consisting of alternate layers of colloidal inorganic microparticles and of an organic colloid, and then removing the alternate monolayers of organic matter so as to obtain a residual coating of layers of colloidal inorganic particles in which all the microparticles are alike.
U.S. Pat. No. 4,131,542, issued to Bergna et al. on Dec. 26, 1978, discloses a process for preparing low-cost silica packing for chromatography comprising (a) spray drying with flowing air at a temperature from 130.degree. C. to 400.degree. C. a silica sol containing from 5-60 weight percent essentially nonaggregated spherical silica particles of uniform size wherein at least 75% of the particles have a diameter of from 0.5 to 2 times the weight average diameter and (b) sintering the resulting porous micrograins to reduce the surface area thereof from 5% to 20%.
Disadvantages of prior art products include coatings which are subject to easy removal as by chipping and flaking, lack of control of variables such as thickness and uniformity of the coating, chemically inhomogeneous surfaces, surface components which are deleterious as catalysts as to certain types of selective adsorption, inability to prepare structures with a uniform surface and with a certain predetermined porosity, and the requirement of many depositions of a single layer at a time. A method which eliminates or minimizes some of these disadvantages is desirable. Moreover it would be advantageous to have the coated materials irreversibly bonded to the core.